In electrical machines with externally or permanently excited armature, the armature, which can, for example, be a rotor of a rotary electrical machine, includes a number of separate armature poles or rotor poles, which alternately form a magnetic north pole and then, following in the direction of armature motion, a magnetic south pole. Two successive armature poles, which form a magnetic north pole and a magnetic south pole and thus a full magnetic period, will also be called an armature pole pair herein.
Particularly, if the number of stator poles per magnetic period of the armature, i.e. per armature pole pair, is an integer number, and if the stator poles, at any time, show a complete and identical arrangement of the stators poles associated to each armature pole pair, a so-called cogging force occurs in the electrical machine. This cogging force is based on non-linear variations of the forces put on the stator poles and caused by the armature poles, which depend on the total of the existing magnetic fields between the armature poles and the stator poles. Non-linear magnetic field portions, which finally lead to the non-linear variations of the force in the electric machine, exist due to the essential gaps between the armature poles, which are in magnetic respect also always generated by leakage flux. From a rotary electrical machine's point of view, these initial radial variations of the force will then be transformed into a tangential force by means of the motion of the armature. In this way the disadvantageous cogging force is generated, which is particularly typical for slow idle motion. Cogging force is also known under the terms “intrinsic cogging performance” and “cogging”. Additionally, the effects described here add up over the total length of the armature, because of the consistent conditions within each single magnetic periods of the armature. The results are uneven running, the necessity of adhering to minimum speed, undesired noise, vibrations etc.
It is typical to use a special geometric design of the stator poles or of the armature poles or of their arrangement with regard to each other in order to reduce the cogging force in electrical machines. Such an approach is described in DE 195 07 490 C2. It turns out, however, that this approach is very costly and that it usually has a negative impact on the efficiency of the electrical machine.
It is known from DE 41 33 723 A1, where a three-phase electrical machine is concerned, that one armature pole pair is assigned to the integer number of three stator poles, as it is common. However, the cogging forces of the single stator poles are displaced to each other by different distances between the armature poles, which has the result of a kind of averaging of the cogging forces over the whole armature instead of an addition. The disadvantage here is that the actual described three-phase electrical machine can only be used for a very narrow range of operation parameters. Furthermore, the non-equidistant arrangement of the armature poles excludes the use of the so-called Hall element-effect devices as rotor position detectors, which is very common in electronically commuted machines, because the non-equidistant arrangement of the armature poles causes a big angular jitter during the position detection of the armature field.
A first electrical machine of the particular type described at the beginning is known from EP 0 291 219 A1. A difference between the total number of stator poles from the total number of rotor poles with a value of ±1 is intended here. In this way, the cogging forces shall be averaged out over the whole armature. However, especially in case of a high number of poles, it turns out that the averaging of the cogging forces put on the poles no longer completely occurs, because mechanical deformations of the armature or the stator, which may be caused by manufacture or load, or interferences in the equidistance of the armature poles or an eccentricity of the stator or the rotor in the electrical rotary machine emphasize the cogging forces in single areas of the armature, which can no longer be compensated completely through the opposing cogging forces in other areas of the armature. Eccentricities of the rotor or the stator, wobbling of the rotor and faulty allocations of the permanent magnets of a permanently excited rotor lead to an undesired intensity of clearly visible cogging torque especially in rotary electrical machines. A relatively high noise level is produced too, which is caused by the characteristic distribution of the cogging forces in the electrical rotary machine and by a basic harmonic vibration frequency of the electrical machine with regard to the entire machine surroundings.
Another electrical machine of the particular type described at the beginning is known from DE 195 11 434 A1. Here, the number of stator poles per armature pole pair shall differ from an integer number. This approach is also used by the design for an electrical machine where the total number of armature poles and the total number of stator poles differs more than ±1 from each other. The necessary winding arrangement of the stator for this is complex and thus difficult to achieve manually as well as mechanically. Moreover, the still existing cogging force, caused, for example, through eccentricities of the rotor or the stator of a rotary electrical machine and similar defects, can be accented in partial areas of the electrical machine in such a way that they cannot be averaged out completely over the entire electrical machine, but result in a distinct total cogging torque.
It is known from WO 94/06192 to construct the stator of an electrical machine using several identical stator segments. The stator segments will be equipped with a shared winding, i.e. a winding, which extends over the single segments. This shared winding can be formed, while the stator segments are still in a one level arrangement side by side and are not yet assembled in the form of a ring-shaped stator. WO 94/06192 does not look onto the reduction of the cogging forces of the electrical machine described there. The stator segments are not manufactured separately from each other with regard to their winding and thus with regard to their electrical and magnetic formation.
Thus, it is a problem to be solved by the invention to provide an electrical machine of the particular type described at the beginning, which does not show a considerable increase in cogging forces, even if mechanical deviations exist, that are caused by manufacture or load. A further problem to be solved by the invention at the same time is to keep the production process for the electrical machine as simple as possible.